Optical coherence tomography (OCT) has undergone considerable technological advances over the last two decades, yet image acquisition speed remains a major limiting factor for its use in retinal imaging. Increased speed enables imaging of larger fields of view at finer spatial and temporal sampling, while also reducing the effects of eye motion. Reduction of eye motion is particularly attractive with adaptive optics OCT (AO-OCT) because the cellular-level lateral resolution afforded by AO makes the system more susceptible to eye motion. In this study we employ a research-grade AO-OCT system that acquires images 25 times faster than clinical OCT and use it to investigate the microscopic retina in the living human eye.

Methods

The Indiana MHz AO-OCT system is based on spectral domain technology. It employs a high-speed 1×4 optical switch in its novel, quad-spectrometer detection channel to achieve 1 million A-lines/s acquisition speed. The design also makes efficient use of the light available for detection. A superluminescent diode (λc=790nm, Δλ=42nm) illuminates the retina providing 5.3 μm axial resolution. The sample channel contains the AO system that dynamically corrects for ocular aberrations across a 6.7 mm pupil to provide diffraction-limited lateral resolution (1.7 μm confocal) and improved signal-to-noise ratio of retinal images. Volumes videos were acquired with the MHz AO-OCT system of all major retinal layers from the retinal pigment epithelium layer to retinal nerve fiber layer at 6 ̊ superior to the fovea. Volumes were 1.1 ̊×1.3 ̊ or 0.5 ̊×0.7 ̊ in size (containing 320 B-scans × 400 A-lines) and acquired at real time rates of 7.8 Hz and in 30 volume sequences (3.84 s).

Results

MHz AO-OCT volumes were successfully acquired on three normal subjects. Retinal images were typically of ~30 dB dynamic range, without averaging, and revealed substantially reduced image blur and distortion due to eye motion compared to earlier generation AO-OCT systems developed at Indiana. The 30 dB dynamic range is comparable to that of clinical OCT and permitted visualization of all major retinal layers. The 7.8 Hz volume rate also captured the flow dynamics of retinal capillaries at different depths in the inner retina.

Conclusions

MHz AO-OCT provides sufficient 3D resolution and sensitivity to image at the microscopic level any major layer of the retina.